Method and circuitry for implementing a differentially tuned varactor-inductor oscillator

ABSTRACT

The present invention generally relates to voltage-controlled oscillators. More specifically, the present invention relates to method and circuitry for implementing a differentially tuned varactor-inductor oscillator. In one exemplary embodiment, the present invention includes an LC tank circuit having a couple of terminals, a first and second capacitors, and a first and second varactors. The first and second varactors are connected in series forming a first and a second node. The first capacitor connects the first node and one terminal of the LC tank circuit. The second capacitor connects the second node and the other terminal of the LC tank circuit. A pair of differential input control signals is applied across the first and the second varactors, respectively, to tune the LC tank circuit thereby generating an oscillator output.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims the benefit of priority under 35 U.S.C. § 119 from the provisional patent application, U.S. patent application Ser. No. 60/184,721, filed on Feb. 24, 2000, which is hereby incorporated by reference as if set forth in full in this document.

BACKGROUND OF THE INVENTION

The convergence of various high speed data communication technologies (e.g., Ethernet, fiber channel, IEEE firewire links) into the gigabit domain has focused the efforts of integrated circuit designers on developing high speed circuit techniques for processing broadband signals. A circuit block that is commonly found in these types of communication applications is a voltage-controlled oscillator (VCO). As the main building block of phase-locked loops (PLLs), the VCO can be found, for example, in clock and data recovery circuits.

The VCO is typically an important element in determining the overall noise performance of a PLL. Since the VCO is the part of the PLL which produces an ac output signal whose frequency is proportional to the input control signal, external unwanted noise affecting the input control signal has an adverse impact on the performance of the PLL. Hence, it is desirable to provide a VCO whose input control signal is less susceptible to noise interference thereby improving the performance of the PLL.

Furthermore, conventional VCOs are constructed to provide a single-ended output signal. FIG. 1 is a simplified circuit diagram showing a conventional design of a VCO. Under this conventional design, the VCO includes an active circuit (or driver) 10 having a couple of terminals A, B, an inductor-capacitor (LC) tank circuit 12 and a couple of variable capacitors (or varactors) 14, 16 connected in series. The LC tank circuit 12 and the varactors 14, 16 are connected in a parallel manner across the terminals A, B. The input control signal V_(tune) is connected to the node between the two varactors 14, 16. As FIG. 1 shows, the input control signal V_(tune) is single-ended.

Due to their single-ended nature, single-ended signals, however, are more susceptible to noise interference. It is, therefore, desirable to provide a VCO implemented in CMOS technology based on an inductor-capacitor (LC) oscillator structure that uses differentially controlled varactors.

SUMMARY OF THE INVENTION

The present invention generally relates to voltage-controlled oscillators. More specifically, the present invention relates to method and circuitry for implementing a differentially tuned varactor-inductor oscillator implemented using CMOS technology.

In one exemplary embodiment, the present invention includes an LC tank circuit having a couple of terminals, a first and second capacitors, and a first and second varactors. The first and second varactors are connected in series forming a first and a second node. The first capacitor connects the first node and one terminal of the LC tank circuit. The second capacitor connects the second node and the other terminal of the LC tank circuit. A pair of differential input control signals is applied across the first and the second varactors to tune the LC tank circuit thereby generating an oscillator output.

Accordingly, in one embodiment, the present invention provides a voltage-controlled oscillator including: an LC tank circuit, a plurality of varactors each having a first and a second terminal; and a plurality of capacitors respectively coupling said LC tank circuit to said plurality of varactors; wherein, a pair of differential input control signals are applied across said first and second terminals of each of said plurality of varactors to tune said LC tank circuit so as to generate an oscillator output.

Accordingly, in another embodiment, the present invention provides a method for implementing a voltage-controlled oscillator including: connecting a first varactor and a second varactor in series thereby forming a first and a second node; coupling a first capacitor between an LC tank circuit and said first node; coupling a second capacitor between said LC tank circuit and said second node; and applying a pair of differential input control signals across said first varactor and said second varactor respectively.

Reference to the remaining portions of the specification, including the drawings and claims, will realize other features and advantages of the present invention. Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified circuit diagram showing a conventional design of a VCO;

FIG. 2 is a simplified circuit diagram of an exemplary embodiment of the present invention; and

FIG. 3 is a simplified circuit layout of the exemplary embodiment as shown in FIG. 2.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The present invention will now be described. FIG. 2 shows a simplified circuit diagram of an exemplary embodiment of the present invention. As shown in FIG. 2, the exemplary embodiment shows a VCO 18 having an active circuit 20 having a couple of terminals X,Y, an LC tank circuit 22, a first capacitor 24, a second capacitor 26, a first resistor 28, a second resistor 30, a first varactor 32 and a second varactor 34. Further, the LC tank circuit 22 includes an inductor 38 and a third capacitor 26. These various elements are connected as follows.

The LC tank circuit 22 is connected across the terminals X and Y of the active circuit 20. The two resistors 28 and 30 are connected in series. The two varactors 32 and 34 are, similarly, connected in series. The two resistors 28 and 30 in series and the two varactors 32 and 34 in series are then connected in a parallel manner forming a first node C and a second node D. The first capacitor 24 is then used as a bridge connecting the first node C and the terminal X; likewise, the second capacitor 26 connects the second node D and the terminal Y. A pair of differential input control signals V_(tune) and V_(tunen) are applied at the nodes between the two resistors 28 and 30 and the two varactors 32 and 34, respectively to achieve tuning of the LC tank circuit 22. FIG. 3 is a simplified circuit layout of the exemplary embodiment as shown in FIG. 2 using CMOS technology.

The operation of the exemplary embodiment as shown in FIG. 2 is described as follows. Referring back to FIG. 2, the two capacitors 24 and 26 are coupled between the LC tank circuit 22 and the first and second node C and D respectively. By their functional nature, the capacitors 24 and 26 block off any DC current into the LC tank circuit 22 from the first and second node C and D. Hence, the two varactors 32 and 34 are, in effect, AC coupled to the LC tank circuit 22.

Since no DC current flows into the LC tank circuit 22, this configuration allows both terminals of the two varactors 32 and 34 to be connected to any arbitrary DC voltage potentials. This is because the pair of differential input control signals V_(tune) and V_(tunen) are not limited to a voltage range within the power supply. Instead, the pair of differential input control signals V_(tune) and V_(tunen) can be provided at any arbitrary DC voltage potentials thereby allowing the voltage across the varactors 32 and 34 to assume any arbitrary DC voltage potential as well. As shown in FIG. 2, differential input control signals V_(tune) and V_(tunen) are applied to the varactors 32 and 34 and the LC tank circuit 22 via AC coupling capacitors 24 and 26. The VCO 18 can thus be differentially controlled which results in significantly improved noise performance.

In one embodiment, the two varactors 32 and 34 are implemented by the gate oxide of an MOS transistor structure inside a well region. Given a p-type silicon substrate, for example, the MOS varactors would be constructed of a polysilicon-gate oxide-n-type silicon sandwich inside an n-well. The n-well which essentially acts as the bottom plate of the varactor provides isolation from the substrate. Alternatively, the two varactors 32 and 34 can also be implemented using junction varactors.

The AC coupling capacitors 24 and 26 each has a fixed capacitance and can be implemented by a number of different capacitor structures including an MOS capacitor, polysilicon-insulator-polysilicon, or metal-insulator-metal. In a preferred embodiment, the AC coupling capacitors 24 and 26 are implemented using the metal-insulator-metal (MIM) type of capacitor as MIM type of capacitors exhibit minimum series resistance. In addition, the MIM capacitor is much more linear as compared to an MOS capacitor. A person of ordinary skill in the art will know of other methods and ways to implement the AC coupling capacitors 24 and 26 and the two varactors 32 and 34.

A significant advantage of the differentially controlled VCO of the present invention is that common mode noise is rejected from the LC tank circuit 22, and therefore does not adversely affect the frequency of the VCO 18. This is particularly advantageous in applications such as PLL circuits where analog circuits share the same substrate with noisy digital circuitry that are in close proximity.

Yet another advantage of the present invention is that the tuning range of the varactors 32 and 34 is expanded since larger differential voltages can be applied as compared to the conventional single-ended implementations.

It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims. All publications, patents, and patent applications cited herein are hereby incorporated by reference for all purposes in their entirety. 

What is claimed is:
 1. A method of tuning an LC tank circuit comprising: providing an LC tank circuit having a first terminal and a second terminal; providing a pair of variable capacitors in series having a first tuning node between the variable capacitors and a first node and a second node at respective ends of the pair of variable capacitors; coupling in parallel between the first node and the second node a pair of resistors in series, the resistors in series having a second tuning node between the resistors; coupling the LC tank circuit in parallel to the pair of resistors in series through a first AC coupling capacitor between the first terminal and the first node and a second AC coupling capacitor between the second terminal and the second node; and applying a differential pair of arbitrary DC voltages to the respective first tuning node and second tuning node.
 2. The method of claim 1, wherein the variable capacitors are varactors.
 3. The method of claim 1, wherein the first AC coupling capacitor and the second AC coupling capacitor are MOS capacitors, polysilicon-insulator-polysilicon capacitors, or metal-insulator-metal capacitors.
 4. A tunable LC tank circuit comprising: an LC tank circuit having a first terminal and a second terminal; a pair of variable capacitors in series having a first tuning node between the variable capacitors and a first node and a second node at respective ends of the pair of variable capacitors; a pair of resistors in series having a second tuning node therebetween, coupled in parallel between the first node and the second node; wherein the LC tank circuit is coupled in parallel to the pair of resistors through a first AC coupling capacitor between the first terminal and the first node and a second AC coupling capacitor between the second terminal and the second node; and wherein the first tuning node and the second tuning node are responsive to an applied differential pair of arbitrary DC voltages.
 5. The tunable LC tank circuit of claim 4, wherein the variable capacitors are varactors.
 6. The tunable LC tank circuit of claim 4, wherein the AC coupling capacitors are MOS capacitors, polysilicon-insulator-polysilicon capacitors, or metal-insulator-metal capacitors.
 7. A differentially tunable voltage-controlled oscillator with improved noise performance comprising: an LC tank circuit comprised of an inductor and a capacitor coupled in parallel at a first node and a second node; a first capacitor coupled to the LC tank circuit at the first node of the LC tank circuit; a second capacitor coupled to the LC tank circuit at the second node of the LC tank circuit; a first variable capacitor and a second variable capacitor each having a first variable capacitor terminal and a second variable capacitor terminal, said first variable capacitor terminals being coupled together, said second variable capacitor terminal of the first variable capacitor being coupled to the first capacitor and said second variable capacitor terminal of the second variable capacitor being coupled to the second capacitor; and a first resistor and a second resistor, each having a first resistor terminal and a second resistor terminal, said first resistor terminals being coupled together, said second resistor terminal of the first resistor being coupled to the second resistor terminal of the first variable capacitor and said second resistor terminal of the second resistor being coupled to the second resistor terminal of the second variable capacitor; wherein a first input voltage is coupled to the first variable capacitor terminal of each variable capacitor and a second input voltage coupled to the first resistor terminal of each resistor to apply an arbitrary DC voltage differential for tuning the LC tank circuit.
 8. The differentially tunable voltage-controlled oscillator with improved noise performance according to claim 7, wherein the variable capacitors are varactors.
 9. The differentially tunable voltage-controlled oscillator with improved noise performance according to claim 7, wherein at least one of the pair of capacitors is a metal-insulator-metal capacitor.
 10. The differentially tunable voltage-controlled oscillator with improved noise performance according to claim 7, wherein each of the pair of capacitors has a fixed capacitance.
 11. The differentially tunable voltage-controlled oscillator with improved noise performance according to claim 7, wherein said oscillator is implemented using CMOS technology.
 12. The differentially tunable voltage-controlled oscillator with improved noise performance according to claim 7, wherein the difference between the pair of input voltages is an arbitrary DC voltage potential.
 13. The differentially tunable voltage-controlled oscillator with improved noise performance according to claim 7, further comprising: an active circuit having a first terminal and a second terminal; wherein the first terminal of the active circuit is coupled to the first node of the LC tank circuit and the second terminal of the active circuit is coupled to the second node of the LC tank circuit. 